1. Field of the Invention
The present invention relates to an optical scanning image forming apparatus such as a laser beam printer, a laser COM system or the like, and more particularly to an optical scanning image forming apparatus comprising an irradiating optical unit for applying a modulated optical beam corresponding to image information onto a photoreceptor member while scanning the same, and an optical beam limiter interdisposed between the irradiating optical unit and the photoreceptor member and including an optical beam passing slit formed along a scanning direction of the optical beam.
2. Description of the Prior Art
In the case of a conventional optical scanning image forming apparatus of the above-noted type, the optical beam is regulated only by means of the optical beam limiter (e.g. European patent application published under No. 0088555A1). That is, according to the construction of the above apparatus, by causing only the optical beam having passed the optical beam passing slit to reach the photoreceptor member, even if the optical beam is scanned with its small vibrations normal to the scanning direction caused by tolerances of the irradiating optical unit, the construction is so arranged to prevent deviations of the image formed on the photoreceptor member in the scanning direction.
However, the above construction has drawbacks to be described hereinafter.
The beam passing the optical beam passing slit of the optical beam limiter is spread out by a diffracting effect of this slit. Therefore, in order to increase the scanning line density by reducing a diameter of the optical beam on the photoreceptor member, the optical beam passing slit, i.e. the optical beam limiter need be disposed adjacent the photoreceptor member.
More specifically, if the diameter of the optical beam normal to the scanning direction on the photoreceptor member and the distance between the optical beam limiter and the photoreceptor member are taken into consideration, a range within which there occurs no blurring in the image on the photoreceptor member due to the diffracting effect of the optical beam passing slit is given by EQU l&lt;W.sup.2 /80 (1)
where
l is a distance between the optical beam limiter and the photoreceptor member,
W is a width of the optical beam passing slit,
.lambda. is a wavelength of the optical beam.
And, within this range, we obtain EQU d.about..lambda..multidot.l/W (2)
where
d is a blurring amount of the image on the photoreceptor member,
The diameter D of the optical beam normal to the scanning direction on the photoreceptor member (this diameter will be referred to hereinafter as a sub scanning direction beam diameter) is expressed by EQU D=W+d (3)
By using the above expressions (2) and (3) with the width W of the optical beam passing slit being a parameter, a relationship between the sub scanning direction beam diameter D and the distance l between the optical beam limiter and the photoreceptor member (will be referred to hereinafter as a clearance) may be obtained.
Taking for an example a semiconductor laser beam often used as the optical beam in this type of optical scanning image forming apparatus, the wavelength thereof given by EQU .lambda.=780.times.10.sup.-6
When this equation is applied to the above expressions (2) and (3), a graph showing the above relationship is plotted in FIG. 6. In this graph, lines denoted by marks `a` through `d` respectively illustrate cases where the width W of the optical beam passing slit is 0.12 mm, 0.10 mm, 0.08 mm and 0.07 mm.
As seen from this graph, for obtaining a sub scanning direction beam diameter of 100 .mu.m required for a printer having a resolving power of 400 dpi, even if the width W of the optical beam passing slit is 70 .mu.m, the clearance l need be less than 3 mm.
On the other hand, in a typical image forming operation, there is a step of development in which a latent image formed on the photoreceptor member by the beam scanning operation is developed by selectively adhering toner thereto.
For this reason, in the conventional construction where the optical beam limiter and the photoreceptor member need be disposed adjacent each other, the toner on the photoreceptor member tends to inadvertently adhere to and thus soil the optical beam limiter. Especially, if the construction is so arranged that the toner to be removed from the photoreceptor member is collected by a cleaner attached to the developing device, the above tendency appears more conspicuously since the photoreceptor member with a considerable amount of toner being adhered thereto passes adjacent the optical beam limiter.